Radiant heat derives from electromagnetic radiation that emanates from matter, or more specifically from the atoms that make up the matter. The atoms are electrically charged with the charge distributed over its volume such that on average an associated tiny electrical field called a dipole exists. Since all atoms in matter vibrate, the dipoles associated with the atoms also vibrate, and thus emanate an electromagnetic field. The frequency of the emanated field is equal to the atom's frequency of vibration, which we characterize as its temperature. Thus, radiant energy from all matter is proportional to its temperature, and all matter with a temperature above absolute zero radiates.
Atoms not only radiate electromagnetic radiation, they also absorb it. This is because atomic charge responds to a superimposed electric field. Therefore, atoms that experience an electromagnetic field with a higher frequency will begin to vibrate at that frequency and therefore manifest a higher temperature. Moreover, since radiation is quickly attenuated inside a given material, almost all the emission, reflection and absorption of radiation occurs at the surface of the material. Thus, a given system, such as a room containing furniture and humans, will include matter of various densities, compositions and temperatures all radiating and absorbing electromagnetic temperature proportional to its temperature.
All electromagnetic radiation travels at the speed of light and generally in straight lines. Therefore, the mathematical characterization of radiant heat accounts for its proportionality to the temperature of the radiating body, the emissive, reflective or absorptive nature of the body's surface, and the geometry of the areas that are “in sight” of each other. This last term is known as the “view factor.” The relationship between the flux of radiant heat and these factors is linear except for temperature, where it proportional to the fourth power (T4).
Rooms are often heated by heating the air that they contain. The air can be heated at a remote furnace, e.g., in the basement, and blown or forced into the room, or the air can be heated by hot water that is piped to a “radiator.” Alternatively, an electric heating element can be energized and air can be caused to flow past the element. The air may flow past these devices either naturally (convective heat transfer) or by being forced by a blower or fan. Therefore, humans feel warmth by feeling the warm air next to their skin and clothing. Of course, the heating of a room is always a combination of convective, conductive and radiative heat transfer so that when one refers to a room's heating mode, it is generally with reference to the predominant heating mode. For example, if the room contains a window, there may be a strong radiative heat transfer component from the sun that constitutes the predominant heating mode during a sunny day.
Rooms can also be heated with a predominant, engineered radiant heat transfer component at all times. Radiant heating systems are characterized by large areas from which heated surfaces radiate and lower temperatures. Rooms can also be heated with engineered radiant heating systems as an added comfort factor. For example, a bathroom may have a forced hot air system for heating the air together with a radiant heated floor to heat bare feet and add a measure of “comfort.” Radiant heating uses the electromagnetic property of materials described above to radiate energy for absorption by human skin and clothing. It is therefore sometimes referred to as “direct heating,” or as “heating the people and not the air.” Moreover, radiant heating can utilize all surfaces in a room because the air in the room is not the predominant absorber of the heat. When air is used to carry heat, there is a tendency for the warmer air, which is less dense, to buoy up to the ceiling where it cannot heat people. However, with radiant heat, the air is heated less and better absorbers, e.g., humans, are heated more. Therefore, one can utilize ceilings, floors, walls and room dividers as surfaces for heat radiation.
In a room heated by radiant heat, there is a heat source located at or below the radiating surface. When the heat energy reaches the radiating surface, e.g., the floor, the air at the surface is heated and rises as cooler air sinks to displace it. However, much of the energy is radiated from the surface to all objects within view that are cooler depending on the emissive properties of the floor material. Clearly, it is advantageous to choose a surface material with high emissivity such as tile rather than one of low emissivity such as carpet, and it is more energy efficient to choose surface treatments for other surfaces such as walls to have covering that have low absorption.
There are two common types of radiant heating systems: hydronic and electric. Hydronic systems consist of water pipes made of either copper or plastic that are placed on a subfloor and under the floor surface. The space between the pipes is sometimes filled with a cementitious material, e.g., gypsum, to improve the thermal conductivity between pipes and floor. Usually, hot water is generated at a source such as a water heater located in a basement and pumped to the piping system under the floor. Hydronic systems are inefficient and complicated because frequently multiple fluid circuits are necessary for one floor since the water becomes too cold to uniformly heat a given floor in a single pass. Hydronic systems often are complex to install because they consist of many valves, manifolds, pumps and fluid controls.
Electric systems typically utilize wires that are laid out on a sub floor, and then covered with the working floor surface. Often, the wires are thin enough such that the cement used to attach the tiles is sufficient to support and protect them. Heat transfer is a problem with electric wire systems because all the heat energy that is ultimately absorbed by humans in the room must be generated along a thin wire. From the wire, the heat must conduct through its sheath, up to the floor, and then laterally across the floor surface if uniform heating is desired. Since a temperature gradient is the driving force for conduction, the wires must operate at high temperature to flow the heat properly to the upper floor surface. Stated another way, the tiny wire surface must generate a very high power density (in watts/in2) in order to achieve even a small energy density (such as 12 watts/ft2) at the floor surface.
A less common electric radiant heating system is the STEP Warmfloor™ system from Electro Plastics, Inc. of St. Louis, Mo. This system utilizes a carbon resistor encapsulated in polymer film that serves as an underlayerment for tiles, carpet and other floor coverings. A similar system is described in U.S. Pat. No. 6,737,611 to Ek et al. These systems exhibits improved efficiency over wire-based heaters, but they have not been widely used due to the high cost of the materials and the difficulties in installing and using these systems for certain applications, such as in non-rectangular and/or irregularly-shaped floors.